Manholes constituting upright cylinders are located at various positions within a sewage system or the like, to define areas of intersection or access to given sewage flow paths as defined by the various pipes of a sewage system. For instance, such vertical wall manhole cylinders exist at points where several pipes merge, as for instance where two or more inlet pipes are connected to a single outlet pipe. Under such circumstances, the cylindrical manhole structure constituting a preformed concrete cylinder which defines a cylindrical internal cavity, is provided with circular openings within the wall of the same at some position above its bottom with the various concrete pipes terminating at the manhole cylinder and with the pipes opening to the interior of the manhole. The ends of the pipes may be flush with the sidewall or projecting slightly into the interior of the cavity defined by the manhole cylinder. In order to permit the inlet flows to merge and to flow through a common outlet, for instance, it is normal to employ wooden or partially wooden forms for defining troughlike connections between the pipes and to fill the interior of the manhole with the exception of the hollow trough forms with concrete, whereupon the forms "known as invert forms", are removed either after the concrete has fully hardened or at least partially hardened, whereupon semicircular cross-sectional intersecting trough flow paths are provided within the hardened concrete, with the tops of the troughs being open. The flow may enter from those pipes constituting the inlet pipes to the manhole, the flow merging at points where the trough paths intersect and continuing to a single outlet pipe circumferentially spaced from the single or multiple inlets.
Such flow paths have been inadequate to say the least since in most cases, the invert form permits the deposition of concrete only to the extent of creating semi-circular cross-sectional flow paths which rise approximately half the height of the pipes entering the manhole interior. Further, there is extreme difficulty in the creation of invert forms which in fact intersect at other than at right angles to each other or in which there is some curvature of flow relative to, for instance, a straight through flow path between diametrically opposed inlet and outlet pipes.
Additionally, while the wooden or plastic and wooden invert forms permit the creation of intersecting flow paths, that is, multiple inlets feeding to a common outlet or vice versa, the invert form assembly is limited to a single use. The form segments are not normally employable without alteration or essential reconstruction within other manholes of the sewage system, and particularly, the form assembly was not applicable to manholes of different diameter, different number and location of pipe openings to the interior of the manhole or for defining flow converging or diverging paths of differing angles of intersection.